Takes an already connected socket and starts client side TLS
handshake to make the communication secure. When the returned
future completes the RawSecureSocket has completed the TLS
handshake. Using this function requires that the other end of the
connection is prepared for TLS handshake.

If the socket already has a subscription, pass the existing
subscription in the subscription parameter. The secure
operation will take over the subscription by replacing the
handlers with it own secure processing. The caller must not touch
this subscription anymore. Passing a paused subscription is an
error.

If the host argument is passed it will be used as the host name
for the TLS handshake. If host is not passed the host name from
the socket will be used. The host can be either a String or
an InternetAddress.

Calling this function will not cause a DNS host lookup. If the
host passed is a String the InternetAddress for the
resulting SecureSocket will have this passed in host as its
host value and the internet address of the already connected
socket as its address value.